1. Field of the Invention
The present invention relates to furnaces, and, more particularly, to induced draft, fuel-fired furnaces having a standing pilot.
2. Description of the Related Art
High efficiency fuel-fired furnaces of the type herein concerned include a combustion chamber wherein fuel such as natural or liquified petroleum (LP) gas is burned creating hot gaseous products of combustion, i.e., exhaust gas. A burner having one or more exhaust ports formed therein is connected to a fuel source and burns the fuel which is ignited. The hot exhaust gas is circulated through a heat exchanger which is heated thereby. Air circulates past the heat exchanger and is heated thereby via convection heat transfer. The heated air is circulated through a living space by a circulating blower. In an induced draft furnace, an exhaust blower is located between the heat exchanger and the flue outlet of the furnace and draws the products of combustion through the heat exchanger and discharges them into an exhaust vent.
To ignite the fuel exhausted from the burner, it is known to use either an electronic ignition apparatus or a standing pilot. An electronic ignition apparatus is operably controlled when fuel is exhausted from the burner to ignite the fuel. An electronic ignition system is considerably more costly and complex than a standing pilot. A standing pilot maintains a continuously burning open flame regardless of whether fuel is exhausted from the burner. During periods of inactivity of the burner, therefore, it is necessary to vent the relatively small amount of combustion gas produced by the standing pilot to the flue and ambient environment. A furnace using a standing pilot is less expensive to manufacture than a furnace using an electronic ignition system and may therefore be preferred.
It is known to vent the combustion products produced by a standing pilot through the heat exchanger and into the flue. However, some high efficiency furnaces provide increased heat exchanger efficiency by increasing the pressure drop of the exhaust gas between the inlet and outlet of the heat exchanger. This may be accomplished by providing a longer flow path and/or appropriate sized conduits at various locations within the flow path. This higher pressure drop increases the efficiency of the heat exchanger when exhaust gas is circulated therethrough, thus requiring less heat transfer area for a given output; however, it also prohibits the venting of the standing pilot combustion products through the heat exchanger during periods of inactivity of the burner, i.e., idle flow conditions.
The higher pressure drop therefore further increases internal heat exchanger resistance prohibiting the safe venting of standing pilot combustion products. Thus, a standing pilot may be preferred to reduce manufacturing costs, but may not be safely vented into the heat exchanger and ultimately to the vent.
One method of obtaining the advantages of using a standing pilot while at the same time avoiding the problems of unsafe pilot gas venting mentioned above is to provide a vent tube disposed immediately above the standing pilot. The vent tube is connected to and in fluid communication with the outlet section of a draft inducing fan. Such an apparatus, however, requires relatively precise placement of the vent tube above the standing pilot flame and a specially manufactured or modified blower. Moreover, to prevent back flow of exhaust gas during operation of the blower, a special venturi section forming member must be rigidly fixed at a precise location within the blower adjacent the outlet of the vent tube, thereby further increasing manufacturing costs.
What is needed in the art is an apparatus and method which allows a standing pilot to be used with a high efficiency furnace by easily and inexpensively bypassing the combustion products of the standing pilot directly to the flue of the fuel-fired furnace.